My computer is set up to require a login after its been put to sleep and it has a fairly complex password, so I'm not worried about anyone guessing it. The MacBook was stolen, and I'm wondering if there's a way for that person to access my files, or will they just have to wipe the drive (hopefully).

It would be trivial for someone to boot using a Linux LiveCD and mount your drive, thus accessing your files. All they'd have to do to get past your sleep/password would be to hard power down the system.

Additionally, from the link above:

Mac OS X: Single User Mode

To boot a Mac into “Single User mode”, simply boot the computer and press Apple + S when blue first
shows up on the screen. Next, mount the harddrive, and either dump the
password and crack it with a tool like John the Ripper, or simply
overwrite the root password:

thanks; it happens. it was a break-in so many things were taken. the most critical stuff was on an encrypted disk image so I think that's safe but at least I know I should consider everything else compromised.
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SedJun 8 '12 at 22:57

@Sed My apologies as well, that's really terrible. Now obviously there's a risk that the thief's will be able to gain access to your files, but more likely than not, they lack the knowledge to do so and your computer will probably end up being wiped clean without anyone even attempting to peak around.
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0x7fffffffJun 10 '12 at 6:48

As Kalamane's answer points out, unfortunately the thief can access your data very easily, as it sits on your disk unencrypted. It is unfortunately trivial to bypass OS X passwords by booting into single user mode.

It won't help now, but for the future, here are two tips that can help in case of laptop theft.

First of all, if you're like me and carry your laptop almost everywhere (so the likelihood of it getting stolen/lost is relatively high), seriously consider Apple's FileVault 2. FileVault 2 encrypts your entire hard-drive, seamlessly. This means that you can still use your computer as normal (you won't even notice that FileVault is on), but in case it gets stolen, the attacker won't be able to read anything from the hard-drive without knowing your password.

Secondly, enable the "Find My Mac" feature available in OS X Lion (go to Preferences - iCloud - Find My Mac). In case your laptop gets stolen, you will then be able to login to icloud.com from any computer, and see the last location where your MacBook has been on a map. You will also get the following interface, which you can use to remotely display a message on your Mac, wipe the Mac completely, or lock it: